Method of making cathodes



April 1946- P. G. CHEVIGNY METHOD OF MAKING CATHODES Filed Feb. 13, 1943IN V EN TOR. HQUL 6' CHE V/GN Y ATTORNEY Patented Apr. 2, 1946 METHOD OFMAKING CATHODES Paul Georges Chevigny, New York, N. Y., assignor toFederal Telephone and Radio Corporation, Newark, N. 1., a corporation ofDelaware Application February 13, 1943, Serial No. 475,737

Claims. (01. 148-131) This invention relates to improvements in methodsof making cathodes, and more particularly to the setting andcarburization of spiral thoriated tungsten cathodes adapted for use invacuum tubes and the like.

The primary object of my invention is to provide a simple method forpre-setting and carburizing a spiral tungsten cathode with a minimumamount of distortion, so that the final carburized cathodes will bedimensioned within relatively accurate limits.

Other objects and advantages of the present invention will appear fromthe following detailed description thereof, taken in connection with theaccompanying drawing in which:

Fig. l is a vertical view illustrating the step of winding a cathode ona mandrel;

Fig. 2 is a vertical view partly in section, showing the cathode in acarburizing sleeve and partially within a heating device;

Fig. 3 is a vertical cross-sectional view showing the mounting of thefinished spiral cathode on its supporting structure.

In accordance with the proposed method of forming spiral cathodes, it isdesirable before carburization, to set the spiral winding in its finalform as much as possible. To effect such setting, the winding III whichmay be of thoriated tungsten wire is wound upon a suitable mandrel l2which is preferably, but not necessarily, formed of molybdenum. Themandrel l2 has an outside diameter equal to the desired inside diameterof the final cathode and the length of the cathode is determined by thenumber of turns wound on the mandrel. The turns may be either closelyspaced, as illustrated in Fig. 1, or more widely spaced, as" desired.After the wire is wound on the mandrel l2 to form the cathode III, themandrel is heated to a temperature of approximately 2000 C. in ahydrogen atmosphere. This heating may be accomplished in any suitablemanner but preferably by the heat ing eflect'of induced high frequencycurrents.

After the cathode III has been set, the spiral structure is slid intothe smooth bore of a graphite sleeve I4. The sleeve is coated with acolloidal solution of graphite in water known as Aquadag, and the wholestructure is dipped into this "Aquadag or coated inside with the samematerial. The graphite sleeve it together with the enclosed cathode I0is then placed into a suitable heating device I! which may be a highfrequency coil supplied with power through leads I! and again heated ina hydrogen atmosphere heat is maintained until a carburization of atleast is obtained. The enclosing graphite sleeve l4 prevents radialexpansion and distorillustrates a preferred arrangement for mountingsuch a pre-set, carburized cathode. I have there shown a plurality ofsupporting rods I6 whose common outer circumference is equal to theinner circumference of the finished cathode. A ring I8 is rigidlyattached near one end of the supporting rods l6, of which there may be,for example, four in number. The spiral cathode I0 is then slid over therods l6 until one end rests against the ring l8. A second ring 20 isthen slid over the rod l6 against the other end of the cathode spiral inand rigidly attached to the supporting rods. The cathode I0 is thusrigidly held in place on the rod l6 against endwise motion between ringsIn and 20 and will. have no radial movement by virtue of the close fitof the spiral about its supporting rods. Within the cathode In there maybe provided a central supporting rod 28 carrying a filament winding 32for energizing the cathode by radiation or by bombardment.

As previously mentioned, while the cathode spiral III has beenillustrated as a closely wound spiral, the invention is equallyapplicable to the making of cathode structures in which the turns arespaced from one another, for example, the turns may be spaced so that acomposite cathode formed of two or more interwound spirals could beformed, with the result that the final cathode to a temperature ofapproximately 2000 C.- This 56 structure would have a substantiallycontinuous emitting surface similar to that of a single closely woundcathode. The formation of a cathode with closely wound turns forming acontinuous emitting surface is particularly advantageous where thecathode is to be energized from an enclosed filament by bombardment.

The particular manner of mounting the cathode made in accordance withthe method of the present invention and described above, forms no partof the present invention, but is more fully described in my copendingapplication S. N. 474,- 968, filed February 2, 1943.

While I have described with some particularity the specific steps inmaking a cathode in accordance with the preferred method of the presentinvention, and furthermore, in-connection with the making of a certaintype of spiral cathode, it is contemplated that the invention isapplicable to the making oi other types 01' spiral cathodes and that themethod may include various changes and modifications as will be apparentto those skilled in this art.

I claim:

1. The method of making a spiral cathode which includes the steps ofwinding the cathode into the desired shape on a mandrel, setting thewinding, removing the set winding from the mandrel and coating it with acolloidal graphite solution, covering the inside of a smooth graphitesleeve with the colloidal graphite solution, sliding the winding intothe sleeve, heating the sleeve in a hydrogen atmosphere until a 40% acarburization of the cathode is obtained, and then removing the cathodefrom the sleeve.

- 2. The method according to claim 1, in which the step of setting thecathode is effected by setting the mandrel in a hydrogen atmosphere at atemperature of approximately 2000' C.

3. The method according to claim 1, in which the sleeve isiheated toabout 2000 C.

4. The method according to claim 1, in which the step of setting thecathode is eflected by heating the mandrel in a hydrogen atmosphere at atemperature of 2000 C., and in which the sleeve is heated to about 2000C.

5. The method of making a spiral cathode, which includes the steps ofwinding the cathode into the desired shape on a mandrel, setting thewinding, removing the set winding from the mandrel'and coating it with acolloidal graphite solution, covering the inside of a smooth graphitesleeve with the colloidal graphite solution, sliding the winding intothe sleeve, heating the sleeve in a hydrogen atmosphere for carburizingthe cathode, and then removing the carburized cathode from the sleeve.

PAUL GEORGES CHEVIGNY.

